1. Field of the Invention
The invention relates to preform elements comprising a facing table of polycrystalline diamond bonded to a substrate of less hard material, such as cemented tungsten carbide.
2. Background of the Invention
Preform elements of this kind are used as cutting elements in rotary drag-type drill bits, and formation-engaging inserts on roller cone and percussive bits. The present invention is particularly applicable to the treatment of such preform elements before they are mounted on the drill bit, although the invention is not restricted to elements for this particular use. Alternatively, preform elements of the kind referred to may be employed in workpiece-shaping tools, high pressure nozzles, wire-drawing dies, bearings and other parts subject to sliding wear, including bearing elements subject to percussive loads such as tappets, cams, cam followers, and similar devices requiring wear-resistant surfaces.
Preform elements of the kind to which the invention relates are generally manufactured by pre-forming a substrate in an appropriate shape from compacted powdered material, applying one or more layers of diamond particles to the surface of the substrate, and then densifying the substrate and diamond layer(s) to form an integral unit. Densification is achieved via a high pressure, high temperature process in a forming press so that the diamond particles bond together and to the substrate by a sintering mechanism. Diamond-to-diamond bonding occurs during densification, producing a polycrystalline diamond composite layer bonded to the substrate. Such elements are commonly referred to as PDC (polycrystalline diamond compact) inserts. High temperature, high pressure manufacturing processes for production of PDC elements are well known and will not be described in detail.
In drag-type drill bits, each preform cutting element may be mounted on a carrier in the form of a generally cylindrical stud or post received in a socket in the body of the drill bit. The carrier is usually formed from cemented tungsten carbide, the surface of the substrate being brazed to a surface on the carrier, for example by a process known as xe2x80x9cLS bondingxe2x80x9d. In the LS bonding process, the diamond facing layer is cooled while the substrate is brazed to the carrier, to avoid heating of the polycrystalline diamond facing table above about 725xc2x0 C., beyond which threshold graphitization and internal fracture reactions can degrade properties. Since high-strength braze filler metals typically entail melting temperatures in excess of this stability threshold, cooling of the preform element is normally required for braze bonding. In some types of cutters for drag-type drill bits, and also in some types of inserts for roller cone bits, the substrate of the preform element is of sufficient axial length that the substrate itself may be secured directly within a socket in the bit body or in a roller cone.
Preform elements used in drill bits are subject to high service temperatures and high contact and bending loads, leading to possible substrate cracking, or spalling or delamination of the polycrystalline diamond facing table. These modes of degradation can cause the separation and loss of diamond from the facing table. In particular, failures are often localized at the interface between the diamond table and substrate. Similar fracture processes are observed in preform elements subjected to repetitive percussive loading, as in tappets and cam mechanisms. Residual stresses arising in the preform element due to forming, brazing, and/or fitting processes are believed to significantly influence the tendency for cracking, spalling and delamination progressions. It has become common practice to heat-treat the preform elements after formation in the press and before mounting on the bit body, in an attempt to reduce or modify residual stresses in the element, and thereby reduce the tendency of the elements to crack or delaminate in use.
One common method of heat treatment for thermal stress relief is to maintain the preform elements at temperatures of up to 500xc2x0 C. for periods of up to 48 hours. However, while this is believed to have some stress-modifying effect, subsequent cracking and delamination of the preform elements are still frequently observed in service.
The present invention provides a preform element having a facing table of polycrystalline diamond bonded to a substrate of cemented tungsten carbide with a cobalt binder. The substrate includes an interface zone with at least 30 percent by volume of the cobalt binder in a hexagonal close packed crystal structure.
The present invention also provides a new form of heat treatment for preform elements, which provides more effective thermal stress management, and also reduces the time cycle for manufacturing each element. According to a first aspect of the invention there is provided a method of treating a preform element having a facing table of polycrystalline diamond bonded to a substrate of less hard material, the method comprising the steps of:
(a) heating the element to a soaking temperature in the range of 550-700xc2x0 C.,
(b) maintaining the temperature of the element in said range for a period of at least one hour, and
(c) cooling the element to ambient temperature.
The substrate may be composed of a cemented tungsten carbide composite, that is to say tungsten carbide particles in a binder phase. The method of this invention, where the temperature of the element is maintained above 550xc2x0 C. for at least an hour, causes microstructural changes within the binder phase near the substrate-diamond table interface which accommodate stress relaxation between the diamond table and the cemented carbide substrate. Reduction of peak internal stress levels increases the threshold loading needed to nucleate and growth crack defects, effectively toughening or increasing the tolerance of preform elements to severe service loading.
In step (a), the temperature of the element may raised to a value in the range of 550-625xc2x0 C., and preferably in the range of 575-620xc2x0 C. In a most preferred embodiment, the temperature of the element is raised to about 600xc2x0 C.
The temperature of the element may be maintained in said range for a period of about one hour, or for a period of at least two hours, depending on the nature of the preform element. In some special cases, it may be advantageous to maintain the temperature of the element in the stipulated range for periods of up to 18 or 36 hours.
In the heating step (a), the temperature of the element is preferable raised to the soaking temperature gradually, for a period in the range of one half to one and a half hours, typically for a period of about one hour.
Steps (a) and (b) are preferably conducted in a non-oxidizing atmosphere.
In the cooling step (c), the temperature of the element is preferably reduced from the soaking temperature gradually, for a period in the range of three to four hours. For example, the element may be allowed to cool gradually to about 200xc2x0 C., then rapidly cooled to ambient temperature.
The method and/or the preform element may be applied to preform cutting elements for rotary drag-type drill bits, where the facing table of the preform element has a substantially flat front face, a peripheral surface, and a rear surface bonded to the front surface of the substrate.
The method and/or the preform element are also applicable to inserts for roller cone bits, where the facing tables of the preform element comprise a range of generally convex shapes. Such shaped facing tables of the preform element may comprise a plurality of polycrystalline diamond layers.
The method according to this first aspect of the invention will reduce the tendency toward substrate cracking and delamination. However, in some cases both of these failure progressions may be further inhibited by subjecting the element to a second, flash heating, step.
According to a second aspect of the invention, therefore, there is provided a method of treating a preform element having a facing table of polycrystalline diamond bonded to a substrate of less hard material, the method comprising a first step of:
(a) heating the element to a soaking temperature in the range of 550-700xc2x0 C.,
(b) maintaining the temperature of the element in said range for a period of at least one hour, and
(c) cooling the element to ambient temperature, followed by the second step of:
(d) heating the element to a temperature above 725xc2x0 C.,
(e) maintaining the temperature of the element above 725xc2x0 C. for a period not exceeding five seconds, and
(f) cooling the element to ambient temperature.
It will be noted that in the second step of the heat treatment the element is heated to a temperature which is greater than the temperature at which the polycrystalline diamond will normally experience degradation due to graphitization or other mechanism. However, according to this aspect of the invention, the temperature is raised above this critical temperature for only a very short period, no more than five seconds. It is found that the activation energy resulting from such brief overheating of the diamond layer is insufficient to initiate graphitization of the diamond, but is sufficient to cause stress-altering plastic deformations which greatly toughens the preform element.
The first steps (a), (b) and (c) of the heat treatment may have any of the parameters referred to above in relation to the first aspect of the invention. Steps (d) and (e) may also be conducted in a non-oxidizing atmosphere.
Preferably in step (d) the element is heated to a temperature above 750xc2x0 C., but below about 850xc2x0 C.
In step (e) the temperature of the element is preferably maintained above 725xc2x0 C. for a period of about four seconds.
The second part of the method, i.e. the steps (d), (e) and (f), may also be advantageous if used alone, without the preceding steps, to relieve residual stress in a preform element.
Accordingly, therefore, the invention also provides a method of treating a preform element having a facing table of polycrystalline diamond bonded to a substrate of less hard material, the method comprising the steps of heating the element to a temperature above 725xc2x0 C., maintaining the temperature of the element above 725xc2x0 C. for a period not exceeding five seconds, and then cooling the element to ambient temperature.